The present invention relates generally to gypsum compositions and methods for preparing gypsum compositions. More particularly, the invention relates to set gypsum compositions with reduced density and to methods for the preparation thereof.
Set gypsum (calcium sulfate dihydrate) is a well-known material that is included comnomly in many types of products. By way of example, set gypsum is a major component of end products created by use of traditional plasters (e.g., plaster-surfaced internal building walls) and also in paper-faced gypsum boards employed in typical drywall construction of interior walls and ceilings of buildings. In addition, set gypsum is the major component of gypsum/cellulose fiber composite boards and products, and also is included in products that fill and smooth the joints between edges of gypsum boards. Also, many specialty materials, such as materials useful for modeling and mold-making that are precisely machined, produce products that contain major amounts of set gypsum.
Typically, such gypsum-containing products are prepared by forming a mixture of calcined gypsum (calcium sulfate hemihydrate and/or calcium sulfate anhydrite) and water (and other components, as appropriate). The mixture is cast into a desired shape or onto a surface, and then allowed to harden to form set (i.e., rehydrated) gypsum by reaction of the calcined gypsum with water to form a matrix of crystalline hydrated gypsum (calcium sulfate dihydrate). It is the desired hydration of the calcined gypsum that enables the formation of an interlocking matrix of set gypsum crystals, thereby imparting strength to the gypsum structure in the gypsum-containing product. Mild heating is utilized to drive off the remaining free (i.e., unreacted) water to yield a dry product.
There is a continuing effort to make many such gypsum-containing products lighter in weight by substituting lower density materials (e.g., expanded perlite or air voids) for part of the set gypsum matrix, for example, in order to enhance acoustical and/or insulation properties as well as handling and transportation efficiencies. However, previous efforts for making gypsum-containing products substantially lighter have not been fully satisfactory because, for example, while significant amounts of foam can be utilized to produce the void content sufficient to achieve lower density gypsum-containing products, such products may, for example, still not achieve the desired level of acoustical and insulating properties. As a result, production of lower density gypsum-containing products has been susceptible to possible adverse effects caused by the relatively high amount of foaming agents provided, while still not achieving results desired in some cases. More recently, gypsum board formed from fibrous calcined gypsum has been disclosed in U.S. Pat. No. 5,041,333. The preparation of fibrous calcined gypsum is time consuming and expensive, thereby making the use of fibrous calcined gypsum to make gypsum board expensive, as well.
Another problem with forming a substantially lower density gypsum-containing product is that dimensional stability can be compromised during its manufacture, processing, and commercial application. For example, in the preparation of set gypsum products, there is usually a significant amount of free (i.e., unreacted) water left in the matrix after the gypsum has set. Upon drying of the set gypsum in order to drive off the excess water, the interlocking set gypsum crystals in the matrix tend to move closer together as the water evaporates. In this respect, as the water leaves the crystal interstices of the gypsum matrix, the matrix tends to shrink from natural forces of the set gypsum that were resisting capillary pressure applied by the water on the gypsum crystals. As the amount of water in the aqueous calcined gypsum mixture increases, lack of dimensional stability becomes more of a problem.
Dimensional stability is also of concern even after the final dried product is realized, especially under conditions of changing temperature and humidity where set gypsum is susceptible to, for example, expansion and shrinkage. For example, moisture taken up in crystal interstices of a gypsum matrix of a gypsum board or tile exposed to high humidity can aggravate a sagging problem by causing the humidified board to expand.
If such dimensional instability could be avoided or minimized, various benefits would result. For example, existing gypsum board production methods would yield more product if the boards did not shrink during drying, and gypsum-containing products desired to be relied upon to hold a precise shape and dimensional proportions (e.g., for use in modeling and mold making) would serve their purposes better.
Accordingly, it will be appreciated from the foregoing that there is a need in the art for a set gypsum composition exhibiting low density without requiring the need to include large amounts of lightweight filler or air voids created by foam. It will be appreciated also that there is a need in the art for such a set gypsum composition of lower density that exhibits enhanced dimensional stability, insulation, and/or acoustical properties. The invention provides such a set gypsum composition and method for the preparation thereof. These and other advantages of the present invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.
The present invention provides a set gypsum composition and methods for the preparation thereof. Advantageously, the set gypsum composition of the invention is of low-density and denomstrates enhanced dimensional stability (e.g., resistance to shrinkage), insulation, and/or acoustical properties.
In one aspect, the present invention provides a gypsum composition prepared by a method comprising forming a mixture using at least calcined gypsum and water. Desirably, the ratio by weight of water to calcined gypsum used to prepare the mixture is at least about 3:1. In this embodiment, the set gypsum composition is formed from, i.e., using, calcined gypsum comprising at least 30 wt. % of non-fibrous calcined gypsum. The mixture is maintained under conditions sufficient for the calcined gypsum to form a set gypsum matrix.
In another aspect, the present invention provides a set gypsum composition prepared by a method comprising forming a mixture using at least calcined gypsum and water, wherein the weight ratio of water to calcined gypsum used to prepare the mixture is at least about 4.5:1. The mixture is maintained under conditions sufficient for the calcined gypsum to form a set gypsum matrix.
In another aspect, the present invention provides a set gypsum composition prepared by a method comprising forming a mixture using calcined gypsum, water, and an enhancing material selected from the group consisting of a trimetaphosphate compound, an ammonium polyphosphate having 500-3000 repeating phosphate units, a polycarboxylic compound, a surfactant, and combinations thereof. In this embodiment of the invention, the weight ratio of water to calcined gypsum used to prepare the mixture is at least about 3:1. The mixture is maintained under conditions sufficient for the calcined gypsum to form a set gypsum matrix.
In yet another aspect, the invention provides a set gypsum composition comprising a continuous phase of interlocking set gypsum matrix. The matrix includes gypsum and evaporated water voids. Desirably, the evaporated water voids volume of the matrix is at least about 69% according to the evaporated water voids volume, xe2x80x9cEWVV,xe2x80x9d test (described herein below). In this embodiment of the invention, the set gypsum composition is formed from, i.e., using, calcined gypsum comprising at least 30 wt. % of non-fibrous calcined gypsum.
In still another aspect, the present invention provides a set gypsum composition comprising a continuous phase of interlocking set gypsum matrix, wherein the matrix includes gypsum and evaporated water voids and wherein the evaporated water voids volume of the matrix is at least about 69% according to the EWVV test. In this embodiment of the invention, the set gypsum composition is prepared using, i.e., it is formed from, an enhancing material selected from a trimetaphosphate compound, an ammonium polyphosphate having 500-3000 repeating phosphate units, a polycarboxylic compound, a surfactant, and combinations thereof.
In another aspect, the present invention provides a set gypsum composition comprising a continuous phase of interlocking set gypsum matrix, wherein the matrix includes gypsum and evaporated water voids, and wherein the evaporated water voids volume of the matrix is at least about 80% (e.g., from about 80% to about 97%) according to the EWVV test.
By way of example, the gypsum compositions of the invention can be provided in the form of a board. The board can be provided in the form of an article, such as, for example, wallboard, a door core, a structural insulated panel system component, an acoustical tile, an insulation panel, or the like. In this respect, the article can also include multiple boards (e.g., as layers). For example, the article can include a second, more dense, gypsum board such that the density of the second gypsum board is greater than the density of the board comprising the gypsum composition according to the invention.